FIELD OF THE INVENTION
The invention lies in the field of power generation. The invention relates to a method for closed-loop output control of a steam power plant with a turbo-generator set having a steam turbine and a generator. In operation, the plant water is injected into or upstream of a superheater heating surface. The invention also relates to a steam power plant for carrying out the method.
A closed-loop output control of a steam power plant and such a plant are disclosed, for example, in published French patent application No. 2 381 172.
Reliable power supply in an electric power supply system presupposes careful balancing between the generation of electrical power by a number of power units and the tapping of the electrical power by a number of consumers in an electricity distribution network. If the generation and tapping of the electrical power are equal, the system frequency, which is an important parameter in an electricity network, is constant. The nominal value of the system frequency is, for example, 50 Hz in the European interconnected network. A frequency deviation that occurs, for example, due to the failure of a power unit and to the connection or disconnection of a consumer, can be regarded as a measure of an increase or decrease in the generator output.
Along with the correction of frequency deviations within a power supply system, a further task is maintenance of a prescribed interchange power at coupling points to subnetworks from which the distribution network (interconnected network or separate network) is assembled. One requirement is, therefore, the availability of a fast increase in the output of a power unit within seconds. A possible response reserve requirement, for example, can be a sudden load increase of approximately 3 to 5% (measured with respect to the full load of the power supply system) within 30 seconds. However, the plant disclosed in French application 23 81 172 is neither configured nor suitable for providing such a fast output reserve.
Pages 18 to 23 of the publication "VGB Kraftwerkstechnik", Issue Jan. 1, 1980, describes possibilities for fast closed-loop output control and frequency back-up control. While a plurality of possibilities of intervention exists that can be carried out simultaneously or alternatively for a fast change in output in the range of seconds (seconds reserve), it is also necessary to change the supply of fuel for a permanent change in the output of a power unit. Therefore, for the purpose of bridging delay times within the first seconds in a fossil-fired steam power plant, it is usual for control valves, held in advance in a throttled position of the steam turbine, to be opened and thereby to activate and discharge available steam accumulators virtually without delay. Such an operation mode of the steam power plant in the throttled state leads, however, to a high proper heat consumption, and is, thus, economical only to a qualified extent.
In addition to an increase in output due to the throttling cancellation of control valves of the steam turbine, it is also possible to shut down feed heaters that are provided in the water-steam circuit of the steam turbine and are heated by extraction steam from the steam turbine. A condensate flow guided simultaneously through the low-pressure feed heater can be stopped within a few seconds and increased again. A measure for fast closed-loop output control in fossil-fired power units by shutting down the feed heaters accompanied by stoppage of condensate is also described, for example, in German Patent DE 33 04 292 C2.
It is conventional to use a governing system to subject the fast seconds reserve to closed-loop and/or open-loop control, in other words, to closed-loop control of the loading of steam flows to regenerative feed heaters and/or heating condensers as well as of the process steam and the condensate in the water-steam circuit of the steam turbine of a power unit.
For fast closed-loop output control, that is to say, activating the seconds reserve, the steam supply to the feed heaters is throttled, throttling the process steam and/or throttling the condensate. In such a case, desired setting values for control valves at the turbine bleed points, and for regulating units for setting condensate, are formed to produce a required extra generator output. However, a configuration of a steam turbine suitable for such purpose is disadvantageous because the configuration is relatively complicated. The closed-loop control mechanism is complex and, therefore, vulnerable, resulting in a system that is reliable for fast closed-loop output control only to a qualified extent.